Among the existing power tools, the impact wrench is used to tighten the screw fastener to work piece. It generally comprises a main shaft driven by the rotation of the motor, impact block connected with the main shaft through spiral scroll and ball, and working shaft fit for the impact block through the end tooth and located in front of the impact block. An impact spring is configured in the back of the impact block, which compresses the impact block to enable the impact block to keep reliable coordination with the working shaft. During working, the rotation movement of the main shaft directly outputs to the screw piece through the impact block and working shaft so as to secure the screw piece to work piece. During the tightening, the load on the working shaft gradually increases. When the load exceeds the preset value, the impact moves toward the motor relative to the working shaft through the rolling of the ball in the spiral scroll, and compresses the spring behind it. At the moment that the impact block and the end tooth of the working shaft are unfitted, under the action of the impact spring, the impact block moves forward in axial direction and beats the working shaft in rotation direction, so as to enable the working shaft to keep tightening the screw piece in the direction of rotation. In such cycles, through endless intermittent beating of the impact block, the screw piece can be secured to a work piece in the end. Electric drill is used to drill holes in work piece. However, the user generally requires tightening screw pieces onto the work piece or drilling holes in work piece during working. In this way, it is very inconvenient when the user is required to change tools again and again for operation.
US patent application No. 2005/0199404A1 discloses a power tool capable of realizing impact wrench and electric drill functions in one tool. The power tool secures the impact block (7) and working shaft (8) on the outer circumference through function shifting mechanism (the function shifting button 33 and connecting piece 25 shown in FIG. 1 and FIG. 4 of this patent for application) to keep the impact and working shaft relatively fixed, in this way, to realize the shifting between impact wrench function and electric drill function. With this structure, the user is only required to adjust the function shifting button to shift between impact wrench function and electric drill function. European patent application No. EP 1050381 A2 discloses another power tool with both impact wrench function and electric drill function. The power tool secures the impact block (5) and working shaft (6) along the axis through function shifting mechanism (Drawing 15, 16, 24, 35 and 36 attached to the patent for application) to keep the impact and working shaft relatively fixed, in this way, to realize the shifting between impact wrench function and electric drill function. However, the power tool disclosed by the abovementioned US patent for application uses a round sleeve with relatively large size as the connecting piece, in this way to increase the overall volume of the power tool, improve manufacturing cost; while the function shifting mechanism of the power tool disclosed by the abovementioned European patent for application requires multiple components' cooperation, and the reliability is reduced due to the complicated structure.